43
Tropical cyclones and storms are more common in the Bay of Bengal. They
severely affect the south Indian coast as compared to that of the Arabian Sea.
According to Koteswaram (1984), there were about 346 cyclones that include 133
severe ones in the Bay of Bengal, whereas the Arabian Sea had only 98 cyclones
including 55 severe ones between the years l891 and l970. These cyclones with
tremendous speed hit the coastline and inundate the shores with strong tidal wave,
severely destroying and disturbing coastal life. However, mangroves like
Rhizophora spp. seem to act as a protective force towards this natural calamity
(McCoy et al. 1996).
A series of experiments carried out by the EqTAP project (Development of
Earthquake and Tsunami Disaster Mitigation Technologies and Their Integration
for the Asia-Pacific Region), funded by the Japanese government, have shown that
mangrove forests and certain other types of coastal vegetation can effectively
reduce the impact of tsunamis on coastlines (Hiraishi and Koike 2001; Dinar 2002;
Hiraishi 2003). Empirical and field based evidence is limited, but analytical models show that 30 trees per 100 m
2
in a 100 m wide belt may reduce tsunami flow
rate by as much as 90% (Hiraishi and Harada 2003). EqTAP recommend using a
coastal green belt to protect homes, as it is sustainable, and much cheaper than
artificial barriers (Hiraishi and Koike 2001; Dinar 2002; Hiraishi 2003). Studies in
Vietnam also demonstrated the usefulness of mangrove forests in coastal protection (Mazda et al. 1997).
Mangrove ecosystem acts as the natural barrier against cyclonic depressions,
surges and erosion activities caused by wave actions. According to Kabir et al.
(2006) the mangrove forests are considered a low-cost and natural form of protection for lands subjected to strong currents and surges. The role of mangroves as natural coast guard may be attributed to certain unique properties of mangroves. There
are many documented studies on the hydrodynamics within mangrove swamps and
their wave attenuation properties. These include the research works undertaken by
Mazda et al. (2005), Mazda et al. (1997), Liu et al. (2003), Wu et al. (2001),
Brinkman et al. (1997) and Massel et al. (1999). Field observations of surface wave
attenuation in mangrove forests were undertaken in both Townsville, Australia and
on Iriomote Island, Japan. High resolution wave gauges were deployed throughout
the mangroves along transects in line with the dominant direction of wave propagation. Data were collected to verify a numerical model of wave attenuation. The
numerical model was based on the fact that surface waves propagating within a
mangrove forest are subject to substantial energy loss due to two main energy dissipation mechanisms: (1) multiple interactions of wave motion with mangrove
trunks and roots; and (2) bottom friction. The dissipative characteristics of the mangrove forest were estimated from physical parameters such as trunk diameter, spatial
density and vegetation structure, which were not necessarily vertically and horizontally uniform. The resulting rate of wave energy attenuation depended strongly on
the density of the mangrove forest, the diameter of the mangrove roots and trunks
and on the spectral characteristics of the incident waves. The numerical model
results were supported by field observations, which showed substantial attenuation
of wave energy within the mangrove forest. Typically, wave energy is attenuated by
Types of Tropical Cyclones
Tropical cyclones and storms are more common in the Bay of Bengal. They
severely affect the south Indian coast as compared to that of the Arabian Sea.
According to Koteswaram (1984), there were about 346 cyclones that include 133
severe ones in the Bay of Bengal, whereas the Arabian Sea had only 98 cyclones
including 55 severe ones between the years l891 and l970. These cyclones with
tremendous speed hit the coastline and inundate the shores with strong tidal wave,
severely destroying and disturbing coastal life. However, mangroves like
Rhizophora spp. seem to act as a protective force towards this natural calamity
(McCoy et al. 1996).
A series of experiments carried out by the EqTAP project (Development of
Earthquake and Tsunami Disaster Mitigation Technologies and Their Integration
for the Asia-Pacific Region), funded by the Japanese government, have shown that
mangrove forests and certain other types of coastal vegetation can effectively
reduce the impact of tsunamis on coastlines (Hiraishi and Koike 2001; Dinar 2002;
Hiraishi 2003). Empirical and field based evidence is limited, but analytical models show that 30 trees per 100 m
2
in a 100 m wide belt may reduce tsunami flow
rate by as much as 90% (Hiraishi and Harada 2003). EqTAP recommend using a
coastal green belt to protect homes, as it is sustainable, and much cheaper than
artificial barriers (Hiraishi and Koike 2001; Dinar 2002; Hiraishi 2003). Studies in
Vietnam also demonstrated the usefulness of mangrove forests in coastal protection (Mazda et al. 1997).
Mangrove ecosystem acts as the natural barrier against cyclonic depressions,
surges and erosion activities caused by wave actions. According to Kabir et al.
(2006) the mangrove forests are considered a low-cost and natural form of protection for lands subjected to strong currents and surges. The role of mangroves as natural coast guard may be attributed to certain unique properties of mangroves. There
are many documented studies on the hydrodynamics within mangrove swamps and
their wave attenuation properties. These include the research works undertaken by
Mazda et al. (2005), Mazda et al. (1997), Liu et al. (2003), Wu et al. (2001),
Brinkman et al. (1997) and Massel et al. (1999). Field observations of surface wave
attenuation in mangrove forests were undertaken in both Townsville, Australia and
on Iriomote Island, Japan. High resolution wave gauges were deployed throughout
the mangroves along transects in line with the dominant direction of wave propagation. Data were collected to verify a numerical model of wave attenuation. The
numerical model was based on the fact that surface waves propagating within a
mangrove forest are subject to substantial energy loss due to two main energy dissipation mechanisms: (1) multiple interactions of wave motion with mangrove
trunks and roots; and (2) bottom friction. The dissipative characteristics of the mangrove forest were estimated from physical parameters such as trunk diameter, spatial
density and vegetation structure, which were not necessarily vertically and horizontally uniform. The resulting rate of wave energy attenuation depended strongly on
the density of the mangrove forest, the diameter of the mangrove roots and trunks
and on the spectral characteristics of the incident waves. The numerical model
results were supported by field observations, which showed substantial attenuation
of wave energy within the mangrove forest. Typically, wave energy is attenuated by
Types of Tropical Cyclones